Unravelling nestedness and spatial pattern in pond assemblages

1. Nestedness is a composite property of many suites of biotas. Such patterns may be driven by dispersal limitation, species-area relationships, hierarchical niche requirements, or occur as an artefact of passive sampling. Despite its widespread occurrence, few studies have explored the factors underlying nested subset structure, and ecological distinctions between nested and non-nested (idiosyncratic) taxa within a region have been largely ignored. 2. Macroinvertebrate assemblages from 45 heathland ponds in south-west England are used to (i) unravel the relative importance of processes driving nested subset structure and (ii) test spatially explicit hypotheses concerning the response of nested and idiosyncratic taxa to parameters shown to structure assemblage-level nestedness. 3. Despite being dominated by taxa with good powers of inter-site dispersal, pond macroinvertebrate assemblages were found to be significantly nested. This nesting was not due to passive sampling, and was best explained by pond area, with habitat parameters and isolation being of secondary importance. 4. The spatial responses of nested and idiosyncratic taxa matched predictions; nested taxa showed strong spatial structure, which was reduced when the effects of pond area and habitat were removed. In contrast, a greater proportion of idiosyncratic taxa were completely spatially random and exhibited weaker responses to factors that structure assemblage level nestedness. 5. Nested and idiosyncratic species tend to differ ecologically: idiosyncratic taxa generally possess broad ecological tolerance and good dispersal capacity, whilst nested species are more likely to have narrow tolerances or limited powers of dispersal. 6. Factors structuring nestedness in ponds can be viewed as probabilistic filters which act to limit the spatial distribution of species with narrow ecological tolerance or low dispersal tendency. Nestedness analysis alone fails to elucidate processes that structure assemblage composition. The additional use of spatially explicit analyses is important if processes that generate nested pattern across a region are to be understood.